Process for the production of a composite wire having an aluminum core and a niobium cover

ABSTRACT

A process for the production of a composite wire having an aluminum core and niobium cover surrounding it, the composite wire being produced from a starting rod having an aluminum core in the form of a rod with a hollow niobium cover surrounding it, which starting rod is drawn down using a drawing aid through a plurality of cold drawings until a strong bond between the aluminum and niobium is obtained and in which the composite so formed is subjected to a surface cold deformation after the removal of the drawing aid. In such a method, the present invention provides an improved manner of producing the starting rod in which a carrier is covered with a layer of niobium through fusion electrolysis after which the carrier is removed and the niobium tube formed in that manner then pushed on an aluminum rod of suitable diameter.

United States Patent Diepers PROCESS FOR THE PRODUCTION OF A COMPOSITEWIRE HAVING AN ALUMINUM CORE AND A NIOBIUM COVER [75] Inventor: HeinrichDiepers, Erlangen-Bruck,

Germany {73] Assignee: Siemens Aktiengesellschaft, Munich,

Germany [22] Filed: June 13, 1974 [21] Appl. No.: 478,962

[30] Foreign Application Priority Data June 22, 1973 Germany 1. 2331925[52] US. Cl. 29/599; 29/624; 29/D1G. 11; 174/ 126 CP {51] Int. Cl H01v11/14 [58] Field of Search 29/599, DIG. l l, 624; 174/126 CP, DIG. 6

156] References Cited UNITED STATES PATENTS 3,644,987 2/1972 Scheffleret a1 29/599 3,648,356 3/1972 Ziemek 29/4741 3,687,823 8/1972Lugscheider et a1. 204/3 3,691,031 9/1972 Lugscheider v 204/39 3,777,36812/1973 Pfister et al 29/599 3,781,982 1/1974 Ziemek et al 29/599 1 June24, 1975 OTHER PUBLICATIONS C. Graeme-Barber et al., TubularNiobium/Copper Conductors for AC Super-Conductive Power Transmission,"Cryogenics, Vol. 12, No. 4, August 1972, pp. 317-318.

Primary ExaminerC. W. Lanham Assistant Examiner-D. C. Reiley, Ill

Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin [57]ABSTRACT A process for the production of a composite wire having analuminum core and niobium cover surrounding it, the composite wire beingproduced from a starting rod having an aluminum core in the form of arod with a hollow niobium cover surrounding it, which starting rod isdrawn down using a drawing aid through a plurality of cold drawingsuntil a strong bond between the aluminum and niobium is obtained and inwhich the composite so formed is subjected to a surface cold deformationafter the removal of the drawing aid. In such a method, the presentinvention provides an improved manner of producing the starting rod inwhich a carrier is covered with a layer of niobium through fusionelectrolysis after which the carrier is removed and the niobium tubeformed in that manner then pushed on an aluminum rod of suitablediameter.

PATENTED Jun 2 -15 Fig. 2

PROCESS FOR THE PRODUCTION OF A COMPOSITE WIRE HAVING AN ALUMINUM COREAND A NIOBIUM COVER BACKGROUND OF THE INVENTION This invention relatesto the formation of composite wires of aluminum core with a niobiumcover surrounding the aluminum in general and more particularly to animproved method of producing a starting rod comprising an aluminum rodsurrounded by a cylinder of niobium which may then be cold formed toobtain a composite wire.

A method for the production of a composite wire having an aluminum corewith a niobium cover surrounding the core is disclosed in US. Pat.application No. 378,423 filed July 12, 1973 and assigned to the sameassignee as the present invention. In the process disclosed therein, aninitial or starting rod is first formed from an aluminum rod serving asa core with a niobium cover surrounding the core. The initial rod isthen drawn down through a plurality of cold drawing steps until a strongbond is obtained between niobium and aluminum. During cold drawing, adrawing aid is used surrounding the niobium cover. The composite objectthen has the drawing aid removed and as a final stage of operations issubjected to a surface smoothing cold deformation.

This disclosed process permits a simple means of producing the compositewire obtaining at the same time good super-conductivity characteristics,in particular, small a-c losses from the niobium cover. This latterfeature is important where the composite wire is to be used in thesuperconductive cable, particularly a-c superconducting cables for whichpurpose niobium has been recognized as being an extremely suitablesuperconductive material. The aluminum at the operating temperature ofthe cables, i.e., about 4.2K is a good electric and heat conductorserving to provide electri' cal stabilization of the superconductiveniobium. Aluminum provides significant advantages, particularly in avery pure form since it is relatively light in weight and at lowtemperatures has a particularly low specific resistance.

In the disclosed process, the production of the initial rod isaccomplished by pushing a round aluminum rod into a niobium tube ofsuitable inner diameter and made of solid niobium. It is an object ofthis invention to provide an improved process of forming the startingrod which is used in the process of this prior patent application toobtain a composite wire. A further object of this invention is, toprovide a starting rod which is improved in comparison with the startingrod disclosed in this prior patent application with regard to itsfurther workability and to the superconductivity characteristics of theniobium cover.

SUMMARY OF THE INVENTION In accordance with the present invention, theformation of the starting rod is accomplished by first covering acarrier with a layer of niobium in the form of a cylin drical coat usingfusion electrolysis. After removal of the carrier, the niobium tube soformed is pushed on to an aluminum rod of similar diameter. The niobiumobtained through fusion electrolysis is very pure and finelycrystalline, with a columnar crystalline structure. This makes it veryeasy to shape mechanically thus making it particularly suitable for usein the above described process for production of composite wire througha series of cold drawing operations. In addition, niobium obtainedthrough fusion electrolysis has excellent superconductivitycharacteristics and in particular, has very small a-c losses. inaccordance with the present invention, the cylindrical layer of niobiumover the carrier is advantageously turned on a lathe prior to removal ofthe carrier.

Disclosed as carriers on which the layer of niobium can be deposited aresuitable metals resistive to the fusion electrolysis electrolyte andwhich react as little as possible with niobium. In addition, such metalsmust have a melting point high enough that they do not melt in themolten electrolyte. Thus, aluminum because of its relatively low meltingpoint of 659 is not suitable since the fusion electrolysis electrolyteis at a temperature of 740C during electrolysis. A particularly suitablecarrier is a carrier made of copper. Copper is resistant to theelectrolyte, has particularly no reaction with the deposited niobium andis easily machinable and easily removed. Typically, the carrier may beremoved by boring it out of the cylindrical casing layer of niobium oreven by dissolving it out chemically.

As disclosed, when copper is used as a carrier, it is desirable afterremoval thereof to remove, using a lathe, a layer approximately 0.1 mmthick from the inside of the niobium case. This results in the removalof a thin interdiffusion layer between the niobium and copper which canbe formed during the deposit of niobium and which might impair thesuperconductivity characteristic of the niobium.

As a particularly advantageous method of carrying out the fusionelectrolysis, a method disclosed in an article by Mellors and Senderoffin Journal of the Electrochemical Society, vol. l l2, pages 266 to 272is recommended. In general, the further processing of the starting rodafter the aluminum rod is inserted within the niobium casing followsthat disclosed in the above referenced patent application.

Various drawing aids are disclosed such as a copper tube surrounding theniobium outer layer. In particular, a tube of unannealed copper isrecommended. This is left on during all of the cold drawing stages andremoved only before final surface smoothing. Removal is done chemically.In well known fashion, in accordance with the strict requirements forcold drawing, drawing oil is also applied to the surface. Also mentionedas suitable drawing aids are a lacquer slip surface or a niobiumpentoxide layer formed by anodic oxidation of the surface of theniobium. Although these may be used, they suffer from the disadvantagethat they must be renewed after only a few drawing operation steps andmust be removed again prior to the final surface smoothing operation.

In accordance with the present invention, it is desirable that thesurface of the aluminum rod be etched be fore it is pushed into theniobium tube obtained by fusion electrolysis. The number of cold drawingsteps necessary to obtain a strong bond between niobium and aluminum canbe determined in individual cases by experimentation. A sufficientlystrong bond is indicated when, during further cross section diminishingoperabe obtained in a single drawing step, it is preferable to employ anumber of drawing steps in order to carefully treat the niobium surface.Even after the firm bonding has taken place, further drawing steps maybe carried out to draw the rod down to a wire of the required size. Itis particularly advantageous that the composite wire in a final surfacesmoothing step be round hammered. This insures a very smooth surface ofthe niobium case and at the same time compacts the niobium. This'in turnleads to a high critical current density for the niobium which isparticularly important during the operation at times when the lowercritical magnetic field H is exceeded. In addition, the round hammeringimproves the bonding between the niobium and aluminum. In the disclosedmethod of round hammering, the composite is rotated about itslongitudinal axis relative to the hammer jaws.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration ofan apparatus for coating the carrier by means of a fusion electrolysis.

FIG. 2 is a longitudinal cross sectional view of an initial rodincluding a niobium cover case into which an aluminum rod is insertedand over which a drawing aid is placed.

FIG. 3 is a schematic illustration of the production of a composite wireor rod to the cold drawing of the initial rod of FIG. 2.

FIG. 4 is a schematic end view illustrating the final step of coldhammering of the composite rod or wire.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 illustratesschematically the apparatus of carrying out the fusion electrolysiscoating of a copper carrier with niobium. In accordance with the presentinvention, there is first placed into a container 1 preferably made ofnickel, the constituance of the electrolyte. As noted above,electrolysis is preferably carried out using the process as described byMellors and Senderoff in the Journal of the Electrochemical Society,vol. 1 12, pages 266 to 272 [1965.] A particularly suitable electrolyteis a eutectic mixture of sodium fluoride, potassium fluoride and lithiumfluoride in which is dissolved potassium heptafluoroniobate [K NbF Thedetailed composition of the electrolyte is by weight as follows: 16.2percent K NbF 26.2 percent LiF, 10.4 percent NaF and 47.2 percent KF. Asillustrated, an electrolyte container I preferably made of nickel issituated within the lower part of a coating chamber 2 which ispreferably made of high grade steel. After this, the coating chamber isevacuated using tubular connectors 3 and 4 whereupon an inert gas suchas argon is passed therethrough. The electrolyte 5 is then heatedthrough the use of an electric resistance heater 6 which surrounds thelower part of the coating chamber 2. It is heated to approximately 740Cto cause the electrolyte to melt. The copper carrier 7 to be coated, forexample, a copper rod having a diameter of about mm and an anode 8, forexample, a sheet of niobium con centrically surrounding the coppercarrier 7 are then put into the molten electrolyte 5. With thisarrangement. the copper carrier acts as a cathode. Prior to im' mersionof the electrodes into the electrolyte, a prelim inary deposit on anauxiliary cathode may be carried out in the manner described in GermanOffenllegungs shrift 2.1 14,555. Furthermore, it is desirable asdescribed in detail in German Offenlegungsshrift 2,115,179 to surroundthe cathode with a fine mesh screen made of a material resistant to thefusion elec trolysis electrolyte and to dispose the anode outside of thescreen. By doing so, disturbing influences of slaglike impurities areavoided. Under certain conditions, these could get in to the fusionelectrolysis electrolyte and have deliterious effects. ThisOffenlegungsschrift further discloses apparatus for carrying out suchelectrolyte coating in more detail than it is shown for reasons ofclarity, in FIG. 1. This apparatus and other similar apparatus is wellknown to those skilled in the art.

A voltage of polarity as shown is then applied to the electrodes withthe voltage of sufficient magnitude to establish a current density ofapproximately 40 ma/cm" on the cathode. With this current density andthe above described electrolyte and with an electrolyte temperature ofapproximately 740C, a thickness of approximately 0.6 pm of niobium perminute is deposited on the copper carrier. Deposition can advantageouslybe continued until a layer of approximately 2 mm in thickness isdeposited on the copper carrier 7 having a diameter of approximately 20mm. The copper carrier preferably will be in the form ofa rod and willbe immersed into the electrolyte to a sufficient depth so that acylindrical layer of niobium having length in the axial direction ofapproximately 50 cm will be formed. After coating, the copper carrier isremoved from the electrolyte, allowed to cool under a protective gas andis then removed from the coating chamber. After this, it is preferredthat the deposited niobium layer be finely lathe turned on the outsidefor the purpose of smoothing its surface. The copper carrier is thenremoved by first drilling a central hole therethrough and then using alathe, turning from the inside toward the outside, to remove the copper.After complete removal of the copper carrier, a 0.1 mm layer of niobiumis preferably removed by the lathe in order to insure removal of anyinterdiffusion area present.

This results in a niobium tube 11 such as that shown on FIG. 2, whichtube has been formed by fusion electrolysis. It will thus have theadvantageous characteristics noted above. The niobium tube 11 is thenpushed onto an aluminum rod having a diameter of approximately 19 mm anda length of 50cm. It is recommended that the aluminum rod 12 be etchedwith hydrochloric acid prior to insertion. Over the outside of theniobium tube which will have a diameter of about 24 mm an unannealedcopper tube having an inner di ameter of about 26 mm and a wallthickness of about 1 mm is pushed for use as a drawing aid. Prior toplacing the tube of copper over the niobium, it should be etched indilute nitric acid for cleaning purposes.

The initial rod so formed is then cold drawn in a number of cold drawingsteps as illustrated in FIG. 3 to reduce it to a composite rod ofsmaller diameter. In a preferred method of carrying out the presentinvention, cold drawing is carried out until the diameter of the niobiumcase 11 is reduced to approximately 2.25 mm. Preferably, this is done byreducing the cross section 10 percent during each drawing step. Adrawing speed during the first step, at which speed the starting rod ispulled through the die 15 in the direction of the arrow 14 of FIG. 3.can be approximately 14 m/minute. Thereafter, for further drawing steps,the speed may be increased to 20 m/minute. For facilitating the drawingoperation, a layer of commercial drawing oil is applied over coppertube. After only two or three drawing steps, a firm bond between theniobium and'aluminum will have "been produced. Also suitable as drawingaids are a lacquer slip surface applied over the niobium case or aniobium pentoxide layer formed by anodic oxidation of the surface ofniobium case. Typical of a lacquer slip surface is on'e made of Zaponvarnish or some other quick drying nitro-cellulose lacquer. However, iflacquer orniobium pentoxide layers are used rather than the copper tube,these layers must be renewed after a few drawing operations. Inaddition, the layers just as the copper tube must be removed prior tothe final surface smoothing operation.

After the final drawing step, the copper covering is removed usingnitric acid. This leaves only the alumi num core 12 and niobium caseforming a composite wire. This composite wire is illustrated by FIG. 4being treated in the final operation of round hammering in a roundhammering machine. The general construction of the round hammeringmachine is illustrated. It includes hammer jaws 16 and 17 supportedresiliently for radial travel within an annular mount 18. The annularmount 18 rotates along with the hammer jaws l6 and 17 inside an annularring 19 which rotates at half the rotary speed of the mount 18 and hasthereon a set of rollers 20. As the hammerjaw l6 and 17 rotate past therollers, they are passed inward in the mount 18 and hit the surface ofthe niobium case 11 of the composite wire. The surfaces of the jaws l6and 17 of the hammers which hit the niobium case have a rounded slot orgroove 2t of substantially semicircular cross section which mayadvantageously be tapered down, at least slightly, in the direction inwhich the composite wire advances. The speed at which the wire advancesbetween the hammer jaws 16 and 17 will be, for example, 2meters/minutes. In this step, the prior outer diameter of 2.25 mmpresent after the last drawing step is hammered down to about 2 mm.Preferably, the hammer jaws l6 and 17 can be adjusted to strike thecomposite structure about 4,500 times a minute while turning around thecomposite structure at several hundred revolutions per minute. Thecomposite structure was turned about its axis l5 times per minute duringthat period. This insures that the composite object continuously changesits position relative to hammer jaws thereby becoming well rounded andsmooth. Moreover, the slow turning also prevents the rapidly rotatinghammer jaws from leaving traces on the surface of the wire as it hitsthe composite wire. From this step, the surface of the niobium case ofthe composite wire, which upon removal of the copper case after thefinal drawing step was somewhat undulated, now becomes completely smooththrough the round hammering.

A finished composite wire manufactured according to the presentinvention was tested to determine its a-c losses in the niobium layer,the contact resistance between the niobium and aluminum, and theresidual resistance ratio of the aluminum, i.e.. the quotient of theohmic resistance of aluminum at 300K and the ohmic resistance ofaluminum at 4.2K. Prior to processing the aluminum rod had a residualresistance ratio of about 2,500. In the finished composite wire theresidual resistance ratio had decreased to a value of about 1200, avalue which is still quite high. Such a residual resistance ratio isquite adequate when the composite wire is to be used in asuperconductive cable and annealing of the composite is not necessary.By eliminating an annealing step, a further advantage is obtained inthat it avoids an undesirable formation of a very brittle intermetallicphase of NbAL, in the contact zone of the niobium and aluminum. Thus,with this process, such an intermediate layer, which is easily formed athigher temperatures, is not present and is not caused to be formedduring the cold forming steps used. Even without an actual annealingtreatment at high temperature, the residual resistance ratio of thealuminum may still be surprisingly increased to a value of about 2.000through a brief heating of the composite wire to between 50 and C forapproximately 5 to 30 minutes.

The contact resistance measured between the niobium and aluminum wasquite low, i.e., about 3 X 10 ohms. cm. This low contact resistance alsoshows how intimate the bond between the niobium and aluminum is. A-Closses of the niobium case as long as the lower critical field of theniobium of approximately l0 A/m is not exceeded are also very low. Withan AC frequency of 50 Hz a loss of somewhat less than 0.] 4w per cm ofwire surface was experienced in the niobium case formed by fusionelectrolysis according to the present invention.

In addition to the advantages described above regarding the goodmechanical deformability and good superconductive characteristics of theniobium case obtained through fusion electrolysis, the process of theinvention has all of the advantages described in connection with theabove referenced application. The cold drawing insures an intimate bondbetween the niobium case and aluminum core. This results in smallcontact resistance between the niobium and aluminum. This is ofparticular and decisive importance for a good electrical stabilizationof the niobium case by the aluminum core. As aluminum has a greatercoefficient of expansion under the action of heat than does niobium,strong bonding between the two materials is essential so that theniobium case does not break away from the aluminum core during coolingdown from room temperature to the operating temperature of asuperconductive cable of approximately 4.2K. In addition, a smoothniobium surface keeping a-c losses of the niobium, which are closelydependent on its surface characteristics, quite small is obtainedthrough the final processing step. Thus, it is clear that the compositewire obtained in accordance with the present invention provides anexcellent wire for use in a conductor in a superconducting a-c cable.

Thus, a process for producing an improved composite wire has beendescribed. Although specific steps have been disclosed, it will beobvious to those skilled in the art that various modifications may bemade without departing from the spirit of the invention which isintended to be limited solely by the appended claims.

What is claimed is:

1. in a low temperature process for producing a composite wire having analuminum core and niobium case surrounding the core without submittingthe composite wire to any temperature above about 100C, in which processan initial rod is produced from a rod-like aluminum core with a niobiumcase surrounding the core after which the initial rod is reduced incross section by a plurality of cold drawing steps, with a drawing aidapplied to and surrounding the niobium case, thereby obtaining anintimate mechanical bond between the niobium and aluminum, with thefinal step comprising a surface smoothing cold deformation carried outafter rier is copper.

3. The method according to claim 2 and further including the step ofremoving a layer of approximately 0.] mm from the inside of the niobiumcase after removal of the carrier.

4. The method according to claim 1 wherein the niobium case is smoothlylathe turned on the outside prior to removal of the carrier.

5. The method according to claim 4 wherein said carrier is copper.

1. IN A LOW TEMPERATURE PROCESS FOR PRODUCING A COMPOSITE WIRE HAVING ANALUMINUM CORE AND NIOBIUM CASE SURROUNDING THE CORE WITHOUT SUBMITTINGTHE COMPOSITE WIRE TO ANY TEMPERATURE ABOVE ABOUT 100*C., IN WHICHPROCESS AN INITIAL ROD IS PRODUCED FROM A ROD-LIKE ALUMINUM CORE WITH ANIOBIUM CASE SURROUNDING THE CORE AFTER WHICH THE INITIAL ROD IS REDUCEDIN CROSS SECTION BY A PLURALITY OF COLD DRAWING STEPS, WITH A DRAWINGAID APPLIED TO AND SURROUNDING THE NIOBIUM CASE, THEREBY
 2. The methodaccording to claim 1 wherein said carrier is copper.
 3. The methodaccording to claim 2 and further including the step of removing a layerof approximately 0.1 mm from the inside of the niobium case afterremoval of the carrier.
 4. The method according to claim 1 wherein theniobium case is smoothly lathe turned on the outside prior to removal ofthe carrier.
 5. The method according to claim 4 wherein said carrier iscopper.